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Engineering of the Energy Level Alignment at Organic Semiconductor Interfaces by Intramolecular Degrees of Freedom: Transition Metal Phthalocyanines M. Grobosch,*,† V. Yu. Aristov,†,‡ O. V. Molodtsova,† C. Schmidt,† B. P. Doyle,§,| S. Nannarone,§ and M. Knupfer† IFW Dresden, D-01069 Dresden, Germany, Institute of Solid State Physics, Russian Academy of Sciences, ChernogoloVka, Moscow district, 142432, Russia, TASC-INFM Laboratory, Area Science Park, BasoVizza, I-34012 Trieste, Italy, Department of Physics, UniVersity of Johannesburg, P.O. Box 524, Auckland Park, 2060, Republic of South Africa ReceiVed: February 25, 2009; ReVised Manuscript ReceiVed: June 8, 2009
We have determined the energy level alignment at interfaces between various transition metal phthalocyanines (MnPc, FePc, CoPc, NiPc, CuPc, ZnPc) and gold using photoemission spectroscopy. We demonstrate that the energy level alignment at these interfaces depends upon the type of transition metal. This offers a route to adjust the hole injection barrier via the choice of otherwise equivalent molecular organic semiconductors. In particular, the interfaces MnPc/Au and CoPc/Au are characterized by a small hole injection barrier, which would be advantageous for applications. I. Introduction The energy level alignment at organic semiconductor interfaces is of tremendous importance for the performance of the corresponding devices. This has led to a large number of studies in order to unravel the fundamental properties of organic semiconductor interfaces.1-9 Very often, the interfaces of organic semiconductors and metal electrodes are characterized by the presence of quite large charge carrier injection barriers, and the presence of such contact resistances as well as routes to Ohmic contacts have been the subject of many investigations.10-15 In this context, means to control and adjust the energy levels at particular interfaces have been put forward, among them the introduction of additional interfacial layers, appropriate pretreatments of the metal contact surfaces, and doping of the organic semiconductor.16-22 While these procedures have been successful in a number of cases, they have in common that they increase the complexity of the device fabrication process which, among other aspects, is unfavorable in view of the anticipated low-cost of organic electronic devices. In this study, we demonstrate using photoemission spectroscopy (PES) studies that an intramolecular degree of freedom, the type of transition metal (TM) in TM-phthalocyanines (TMPc’s), can be successfully used to adjust the energy level alignment at interfaces to Au without having to increase the number of fabrication steps of the contacts. Phthalocyanines are members of a large class of organic semiconductors and are already used in organic devices.23-25 The TM-phthalocyanines (TM ) Mn, Fe, Co, Ni, Cu, Zn) all have the same molecular structure and symmetry (D4h) in common;23 they are planar molecules with the TM in the center surrounded by the Pc ligand. Also, their crystal structures are practically identical,26-28 and thin semiconducting films of TM-Pc’s can be grown of equivalent quality independent of the transition metal.29-35 In our study we take advantage of the fact that the electronic properties of the TM-Pc’s near the energy gap is increasingly †
IFW Dresden. Russian Academy of Sciences. § TASC-INFM Laboratory. | University of Johannesburg. ‡
influenced by the metal 3d states of the central TM. While for the late TM-Pc’s (ZnPc, CuPc, NiPc) important properties such as the ionization energy or the character of the highest occupied molecular orbital (HOMO), which forms the valence band in an organic film or crystal, are exclusively determined by the ligand π states, the metal 3d states come very close to the energy of the HOMO and the lowest unoccupied molecular orbital (LUMO) when going to CoPc, FePc, and MnPc, and there is also an increasing hybridization of the ligand and TM wave functions.23,36,37 A direct consequence of this variation is a decrease of the oxidation potential (ionization energy) as probed via electrochemistry, and the first oxidation states of MnPc and FePc for instance have been assigned to metal oxidation.23 One now can expect that such changes of the ionization energy are also reflected in the interfacial electronic properties of the materials when in contact to metals because of the equivalency of the TM-Pc’s in many other respects. II. Experimental The valence band photoemission measurements were performed using different photon energies. We have used a commercial PHI 5600 spectrometer (base pressure 1 × 10-10 mbar), which is equipped with a He-discharge lamp providing photons with 21.21 eV. The total energy resolution of the measurements, determined by analyzing the width of the Au Fermi edge, were about 0.1 eV. The soft X-ray photoemission (PE) spectroscopy measurements were performed also in the ultra high vacuum electron spectrometers at high energy resolution dipole beamlines: the BEAR of ELETTRA (Trieste) and the RGBL of BESSY (Berlin). These beamlines provide continuous radiation with distribution over a wide photon energy (EPh) range of 30-1500 eV. Besides, they are especially suited for studies of fragile organic materials, which potentially could be damaged under X-ray irradiation. The pressure in the experimental systems during data acquisition was always in the range of 1.5-3 × 10-10 Torr. The total instrumental resolution in the PE studies at synchrotron radiation sources (full width at half-maximum, fwhm) also accounting for the thermal broadening was about 130 meV for the VB. The Fermi level (EF)
10.1021/jp901731y CCC: $40.75 2009 American Chemical Society Published on Web 07/01/2009
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Figure 1. Valence-band photoemission data of MnPc deposited onto polycrystalline gold as a function of the MnPc film thickness. The data were taken with a photon energy of 21.2 eV.
position in the spectra was determined after each move of the monochromator by measuring the Fermi edge and/or the Au 4f7/2 electron emission from a gold foil in electrical contact with the TM-Pc film. Thin films of the TM-Pc’s with different thicknesses were deposited by in situ vacuum evaporation in a preparation chamber (base pressure approximately 2 × 10-10 mbar) on polycrystalline or single crystal Au (001) surfaces. Prior to organic film deposition, the gold surfaces were cleaned by Ar+ion sputtering. In case of the single crystal substrates, the surface was prepared by repeated sputtering and annealing cycles after which a 5 × 20 surface reconstruction was observed using low energy electron diffraction, while no remaining contamination was detected in the core level photoemission spectra. The TMPc films were grown by sublimation of phthalocyanine powder at about 500 °C from an effusion cell in the sample preparation chambers. The deposition rate, monitored by a quartz microbalance, was about 1-2 Å/min, while the maximum thickness of the TM-Pc films was about 70 Å as determined by the quartz microbalance and the attenuation of the intensity of the Au 4f substrate peaks,38,39 using an electron mean free path according to the work of Seah and Dench.39 This thickness appears to be large enough to minimize contributions from the gold substrate in the PE spectra and small enough to as much as possible avoid charging effects. For further experimental details, we refer the reader to previous publications.40,41 III. Results and Discussion In Figure 1, we show the evolution of the valence band spectra of MnPc deposited on polycrystalline Au as a function of film thickness taken with a photon energy of 21.21 eV. Equivalent data have been measured for all TM-Pc’s discussed here. From these data one can derive important interface parameters, such as the interface dipole (∆), the hole injection barrier (Φbh), and the ionization energy (IP) of the organic films as is widely described in the literature.1-9 Figure 1 reveals the appearance of the typical valence band structures of MnPc with increasing film thickness, whereas photoemission from the gold substrate becomes weaker and virtually invisible for the thickest MnPc films. Furthermore, there is only a slight energy shift of the MnPc valence band features by about 0.1 eV as a function of film thickness, which shows that interface effects are confined to a very thin interfacial layer. In Figure 2, we summarize the
Grobosch et al.
Figure 2. Evolution of the interface dipole ∆, the ionization energy IP, and the hole injection barrier Φbh at TM-Pc/Au interfaces.
results of our studies that are relevant for the discussion below. These are the interface dipole which is given by the change of the secondary electron cutoff (not shown), the ionization potential, and the hole injection barrier at the TM-Pc/Au interfaces as determined from the energy distance of the onset at the lowest binding energy to the chemical potential (binding energy 0 eV); see also Figure 2. The interface dipole changes only a little throughout the whole TM-PC’s series studied here; there is a tendency to smaller values going from ZnPc to MnPc. This might be related to the close similarity in the molecular structures and thus also the local arrangement of the molecules at the respective interfaces. In contrast, the ionization energies and in particular the hole injection barriers as determined using photoemission spectroscopy vary significantly. While for ZnPc, CuPc, NiPc, and FePc this hole injection barrier is rather large and quite similar (0.8-1.1 eV), it is substantially reduced in the case of CoPc and MnPc (0.2-0.35 eV), i.e. via the choice of the transition metal, the energy level alignment can be adjusted. Our results suggest that it is much easier to inject holes into MnPc and CoPc as compared to the other TM-Pc’s. This however does not necessarily lead to improved charge transport, since transport of carriers also depends on the character of the electronic wave functions, that harbor the injected hole, and their spatial overlap. If this energy level is essentially transition metal 3d-like and thus very localized in the center of the molecule, the holes injected into these states might not be able to contribute to charge transport, but just remain on the molecules at the interface. Consequently, it is essential to derive information on the character of the valence states close to the Fermi level in order to derive relevant information. In the following, we demonstrate that the first ionization state of CoPc is indeed of almost pure Co 3d character and thus strongly localized in the center of the CoPc molecule, while the situation is different with respect to the photoemission for MnPc. Using potential energy surfaces (PESs) it is possible to discriminate between different orbital contributions to a molecular level due to the photoemission cross section variation of the orbital contributions as a function of the applied photon energy. In particular, the C 2pz and N 2pz orbitals forming the π system of the phthalocyanine ligand (the z axis is taken perpendicular to the Pc molecular plane) and the metal 3d states are substantially different in regard of the photoemission cross section. While for a photon energy of 21.21 eV the photoemission spectra to a large extent represents π states (emission from
Transition Metal Phthalocyanines
J. Phys. Chem. C, Vol. 113, No. 30, 2009 13221 IV. Summary
Figure 3. Comparison of the valence band photoemission spectra of MnPc, FePc, and CoPc taken with photon energies of 21.2 and 110 eV (MnPc, CoPc) or 100 eV (FePc). The thickness of the TM-Pc films was about 70 Å.
these states is by a factor of 3-4 per electron in the corresponding orbital more likely than that from metal d-states), the cross sections at 100-110 eV favor emission from the 3d states by a factor of 5-8.42,43 In Figure 3, we compare the valence band photoemission spectra of CoPc, FePc, and MnPc taken at these photon energies. We have normalized the spectra for each Pc to the intensities of the ligand HOMO level, since this level is of a1u symmetry and therefore does not hybridize with any of the metal 3d states. Figure 3 clearly shows the difference for the first ionization states of the three phthalocyanines. For CoPc the photoemission intensity from this state is negligible at a photon energy of 21.21 eV but reaches about 2/3 of that of the a1u ligand orbital at hν ) 110 eV. The situation is very different for MnPc, where the lowest ionization state is clearly visible for both photon energies, but grows with increasing photon energy. For FePc however, intensity changes are observed at the high binding energy side of the first emission feature in direct agreement to what has been published previously.44,45 Consequently, while for FePc the electronic states at lowest binding energy can be assigned to ligand a1u states, the situation is quite different for MnPc and CoPc. We attribute the photoemission feature at the lowest binding energy to a Mn 3d state with eg symmetry, since this is the only metal 3d state that can hybridize with ligand π* states36 and only an admixture of these π* states can explain the observed relatively strong photoemission intensity at hν ) 21.21 eV. In contrast, the lowest lying state in CoPc stems from a different 3d orbital, which by symmetry cannot mix with ligand π states. Indeed, recent studies of CoPc using orbital mediated tunnelling and photoemission have provided evidence that there is a Co 3d state with a1g symmetry 0.4 eV above the ligand a1u orbital,46 in very good agreement to our photoemission data. These studies also confirm that this state is not visible in photoemission studies with a photon energy of hν ) 21.21 eV. In addition, recent density functional theory (DFT) based calculations of MnPc including local electron-electron correlations (DFT + U) predict a highest molecular orbital, which is distributed over the entire MnPc molecule and which arises from a hybridization of metal and ligand eg states37 in direct correspondence to our conclusions. Moreover, this state is expected to be about 0.75 eV above the a1u ligand orbital, also in very good agreement to our photoemission results.
In summary, we have studied the electronic properties of interfaces between transition metal phthalocyanines and gold. Our results demonstrate that the transition metal center has a strong influence on the electronic properties of the phthalocyanine films as well as their interfaces with gold. In particular, the energy level alignment, which has direct consequences for the contact resistance of these interfaces, can be chosen in a large energy range by the choice of the appropriate TM-Pc, and the interfaces CoPc/Au and MnPc/Au are characterized by rather small hole injection barriers. These are directly related to the presence of metal 3d states closest to the chemical potential; a fact that is also reflected in smaller ionization potentials for MnPc and CoPc as compared to other phthalocyanines. Further, we have discussed the nature of the molecular orbitals (metal 3d or ligandlike), that form the states closest to the chemical potential, and it results that they differ between MnPc and CoPc. In CoPc they are of predominantly metallic 3d character with a1g symmetry and do not hybridize with the ligand π system. Therefore, they are most likely to be highly localized due to the very small overlap between these orbitals of adjacent molecules and thus do not contribute to charge transport. Oppositely, the relevant states in MnPc strongly hybridize with the ligand, and thus, injection into these states with a small barrier from gold should also result in continuous charge transport across the interface. Acknowledgment. This work was supported by the DFG (KN393/5 and KN393/9) and by the RFBR (Grant No. 08-0201170). We thank R. Hu¨bel, R. Scho¨nfelder, and S. Leger for technical assistance. The following PACS numbers are associated with this work: 73.20.-r, 78.70.Dm, 79.60.Fr. References and Notes (1) Ishii, H.; Sugiyama, K.; Ito, E.; Seki, K. AdV. Mater. 1999, 11, 605. (2) Salaneck, W. R.; Seki, K.; Kahn, A.; Pireaux, J. J. Conjugated Polymer and Molecular Interfaces: Science and Technology for Photonic and Optoelectronic Applications; Marcel Dekker: New York, 2002. (3) Kahn, A.; Koch, N.; Gao, W. J. Polym. Sci., Part B 2003, 41, 2529. (4) Salaneck, W. R.; Fahlman, M. J. Mater. Res. 2004, 14, 1917. (5) Knupfer, M.; Peisert, H. Phys. Stat. Solidi A 2004, 201, 1055. (6) Cahen, D.; Kahn, A.; Umbach, E. Mater. Today 2005, 8, 32. (7) Knupfer, M.; Paasch, G. J. Vac. Sci. Technol. A 2005, 23, 1072. (8) Koch, N. ChemPhysChem 2007, 8, 1438. (9) Koch, N. J. Phys.: Condens. Matter 2008, 20, 184008. (10) Shen, Y.; Hosseini, A. R.; Wong, M. H.; Malliaras, G. G. ChemPhysChem 2004, 5, 16. (11) Sirringhaus, H. AdV. Mater. 2005, 17, 2411. (12) Hulea, I.; Russo, S.; Molinari, A.; Morpurgo, A. F. Appl. Phys. Lett. 2006, 88, 113512. (13) Maeda, T.; Kato, H.; Kawakami, H. Appl. Phys. Lett. 2006, 89, 123508. (14) Cornil, J.; Bredas, J.-L.; Zaumseil, J.; Sirringhaus, H. AdV. Mater. 2007, 19, 1791. (15) Vazquez, H.; Dappe, Y. J.; Ortega, J.; Flores, F. J. Chem. Phys. 2007, 126, 144703. (16) Blochwitz, J.; Pfeiffer, M.; Fritz, T.; Leo, K. Appl. Phys. Lett. 1998, 73, 729. (17) Gao, W.; Kahn, A. J. Phys.: Condens. Matter 2003, 15, S2757. (18) Koch, N.; Duhm, S.; Rabe, J. P.; Vollmer, A.; Johnson, R. L. Phys. ReV. Lett. 2005, 95, 237601. (19) Kim, W.-K.; Lee, J.-L. Appl. Phys. Lett. 2006, 88, 262102. (20) Yan, X.; Wang, J.; Wang, H.; Wang, H.; Yan, D. Appl. Phys. Lett. 2006, 89, 053510. (21) Rentenberger, S.; Vollmer, A.; Zojer, E.; Schennach, R.; Koch, N. J. Appl. Phys. 2006, 100, 053701. (22) Stadlhober, B.; Haas, U.; Gold, H.; Haase, A.; Jakopic, G.; Leising, G.; Koch, N.; Rentenberger, S.; Zojer, E. AdV. Funct. Mat. 2007, 17, 2687. (23) Leznoff, C. C.; Lever, A. B. P. Phthalocyanines: Properties and Applications; VCH Publishers: New York, 1993.
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